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AP Chemistry 8.2 pH and pOH of Strong Acids and Bases Study Notes

AP Chemistry 8.2 pH and pOH of Strong Acids and Bases Study Notes - New Syllabus Effective fall 2024

AP Chemistry 8.2 pH and pOH of Strong Acids and Bases Study Notes- New syllabus

AP Chemistry 8.2 pH and pOH of Strong Acids and Bases Study Notes – AP Chemistry –  per latest AP Chemistry Syllabus.

LEARNING OBJECTIVE

Calculate pH and pOH based on concentrations of all species in a solution of a strong acid or a strong base.

Key Concepts: 

  • Relative Strength of Acids & Bases

AP Chemistry-Concise Summary Notes- All Topics

AP Chemistry-Concise Summary Notes- All Topics

8.2.A.1  Ionization of Strong Acids in Water:

1. Complete Ionization of Strong Acids:

Strong acids completely ionize in water, which means that they dissociate in their entirety to yield hydronium ions (H₃O⁺) and their conjugate base.

For a strong acid HA, the ionization in water is:

HA(aq)→H3O+(aq)+A−(aq)HA (aq) \rightarrow H_3O^+ (aq) + A^- (aq)* Complete dissociation: All the acid molecules ionize.
* [H₃O⁺] = [HA]₀: The number of hydronium ions formed is equal to the initial number of acid molecules.

 Key Points:

* Strong acids such as HCl, HNO₃, and H₂SO₄ fully ionize in water.
* The hydronium ion concentration [H₃O^+] will be the same as the initial acid concentration.

2. pH Calculation:

i. pH Calculation for Strong Acids:

For strong acids, which completely ionize in water, the pH can be calculated simply with the initial concentration of the acid.

ii. Formula:

pH=−log⁡[H₃O+]

Since strong acids dissociate completely:

[H₃O+]=[Acid]0[\text{H₃O}^+] = [\text{Acid}]_0iii. Steps:

1. Measure the initial concentration of the strong acid (e.g.,[HA]0).
2. Plug the concentration into H₃O+, which is the same as the initial acid concentration.
3. Determine pH: Use the equation pH=−log⁡[H₃O+]\text{pH} = -\log [\text{H₃O}^+].

8.2.A.2  Dissociation of Strong Bases in Water:

1. Complete Dissociation of Strong Bases:

i. Total Dissociation of Strong Bases:

Strong bases, especially Group I and Group II metal hydroxides, completely dissociate in water, producing hydroxide ions (OH⁻).

ii. Dissociation Reaction:

For a Group I base (e.g., NaOH):

For a Group II base (e.g., Ba(OH)₂):

iii. Key Points:

* Total dissociation: All units of the base break apart into ions.
* [OH⁻] = number of OH⁻ ions × [base]₀

* For NaOH:
[OH⁻] = [NaOH]
* For Ba(OH)₂:
[OH⁻] = 2 × [Ba(OH)₂]

2. OH⁻ Concentration and pH Calculation:

The OH⁻ ion concentration is a function of the strength and stoichiometry of the base:

i. Determining OH⁻ Concentration:

* Group I hydroxides (e.g., NaOH, KOH):
Dissociate 1:1 →

[OH−]=[Base]0

* Group II hydroxides (e.g., Ca(OH)₂, Ba(OH)₂):
Dissociate 1:2 →

[OH−]=2×[Base]0

ii. pOH and pH Calculations:

Once you know [OH−][\text{OH}^-]:

pOH=−log⁡[OH−]

\text{pOH} = -\log [\text{OH}^-]pH=14−pOH\text{pH} = 14 – \text{pOH}

 

Strong Acids

  • Strong acids dissociate completely → reaction will go to completion
  • [H+] = [HA] → molarity of H+ = molarity of the strong acid
  • Strong Acids to know: HI, HClO4 (Perchloric), HCl, HBr, H2SO4 (Sulfuric), HNO3 (nitric)
    • Any other acid is a weak acid and won’t dissociate completely
  • [OH-] is going to be small because of equilibrium
  • If [HA] < 10⁻⁷ → water is the dominant acid and donates most of the H+ → pH = 7!
    • Only for strong acids

Strong Bases

  • Strong Bases to know: Group 1 hydroxides, Ca(OH)2, Ba(OH)2, Sr(OH)2 → “CaBaSr”
  • Can tell that it is a base when given equation and it accepts H
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